feat(udpscope): per-element timestamp reconstruction from UDPS frames

Implements FrameDecoder with five timing rules that mirror
UDPSourceSession.cpp: FullArray (per-element time signal), FirstSample
and LastSample (rate-spread from anchor), accumulated scalar with
declared rate (forward-chain anchoring, immune to arrival jitter), and
PACKET burst (backward-span from previous arrival). 9 new tests, 38
pass total.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Client/udpscope/CMakeLists.txt

@@ -83,6 +83,7 @@ set(CORE_SOURCES
     Decimate.cpp
     PaneTree.cpp
     TimeBase.cpp
+    FrameDecoder.cpp
 )
 
 add_library(udpscope_core STATIC ${CORE_SOURCES})